1. Field of the Invention
The present invention relates to a method of a chemical vapor deposition, where a reactant raw material gas(es) is transported onto a semiconductor crystalline substrate(s) to grow a semiconductor crystalline thin film(s) thereon, and to a reactor therefor.
2. Description of the Prior Art
A horizontal chemical vapor deposition reactor, as shown conceptually in a simplified form in FIG. 6 is comprised of a cold-wall type reaction chamber 3 being composed of a gas inlet 5 for a reactant raw material gas(es) 4 at one end and a gas outlet 6 at the other end being aligned horizontally and holding a semiconductor crystalline substrate(s) 1 (hereafter simply referred to as a substrate) roughly horizontally in the chamber 3, and grows a desired semiconductor thin film(s) 2 on the substrate(s) 1, while the substrate(s) 1 is heated and the reactant raw material gas(es) 4 is flown through the substrate(s) 1 in one direction in the reaction chamber 3.
In a horizontal reactor for a chemical vapor deposition of the prior art, as shown conceptually in a simplified form as well in FIG. 7, which is a front view of the chamber 3 in section shown in FIG. 6 as seen from the side of the gas inlet 5, the internal height H of the chamber 3 is so determined as to accommodate a susceptor (not shown) laid on the bottom of the reaction chamber 3 on which a substrate(s) 1 is held and a mechanical driving device for a substrate transportation (both not shown).
With respect to the internal width W of the chamber 3 as shown in FIG. 7 it is so determined that a proper gap is added to a diameter of a susceptor (not shown) or a diameter D of a substrate(s) 1 thereon for receiving or drawing out.
In a chemical vapor deposition using a traditional type of the reaction chamber 3 designed in. such a manner as described above, when, in particular, a low impurity content semiconductor thin film(s) 2 is grown on a substrate(s) 1 having a high impurity content whose concentration is higher than that of the thin film(s) 2 by at least two figures, a transition width T located at the interface between the substrate 1 and the semiconductor thin film 2, where the impurity level in the semiconductor thin film 2 changes gradually from the concentration of the substrate 1 to a desired concentration, is unfavorably spread and thus many trials have been carried out to reduce the transition width T.
To find a better condition for minimizing the transition width T, there have to be done a study about the causes of the transition width T. Traditionally two causes have been raised for the formation of the transition width T, which are out-diffusion O in a solid and autodoping A.
The out-diffusion O in a solid is a phenomenon that an impurity diffuses into a semiconductor thin film 2 from a substrate 1 depending on a growth temperature of the semiconductor thin film 2.
This phenomenon is always dependent on impurity concentration, heating temperature and heating time.
To diminish the phenomenon, one of two ways is chosen, either lowering temperature or shortening heating time.
However, when the heating temperature is lowered in order to suppress the out-diffusion O, automatically affecting to the growth rate to be lowered by itself, the growth rate is required to be reduced further to prevent the surface appearance of a crystalline thin film from roughening.
Under the conditions, the growth time (in other words, the heating time extended to attain a desired thickness of the film) causes not only the enhancement of the out-diffusion in a solid, which reduces the suppression effect of the out-diffusion, but also the reduction of the production efficiency.
On the other hand, the autodoping A is a phenomenon that an impurity moved out from the substrate 1 into the gas phase therearound is incorporated back into the growing surface of a semiconductor crystalline thin film 2.
This phenomenon is also able to be suppressed by either lowering heating temperature or by shortening heating time.
But, both of the means are not better ways to be adopted, because as same as the case of the out-diffusion O mentioned above, both of the means give only small suppression effect to the autodoping A, and also cause a decrease of the production efficiency by lowering the growth rate.
In company with the recent general trend of increasingly higher integrated electronic devices using the semiconductor single crystalline thin films produced by a gas phase epitaxial deposition, much thinner semiconductor crystalline film has been required, and more recently, there has arose a new requirement that the film thickness is equal to or thinner than the transition width T of generally available in the past.